In Situ Raman Spectroscopy of Copper and Copper Oxide Surfaces during Electrochemical Oxygen Evolution Reaction: Identification of CuIII Oxides as Catalytically Active Species
Scanning electron microscopy, X-ray diffraction, cyclic voltammetry, chronoamperometry, in situ Raman spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES) were used to investigate the electrochemical oxygen evolution reaction (OER) on Cu, Cu 2 O, Cu(OH) 2, and CuO catalysts. Aqueous 0.1 M KOH was used as the electrolyte. All four catalysts were oxidized or converted to CuO and Cu(OH) 2 during a slow anodic sweep of cyclic voltammetry and exhibited similar activities for the OER. A Raman peak at 603 cm –1 appeared for all the four samples at OER-relevant potentials, ≥1.62 V vs RHE. This peak was identified as the Cu–O stretching vibration band of a Cu III oxide, a metastable species whose existence is dependent on the applied potential. Since this frequency matches well with that from a NaCu III O 2 standard, we suggest that the chemical composition of the Cu III oxide is CuO 2 – -like. The four catalysts, in stark contrast, did not oxidize the same way during direct chronoamperometry measurements at 1.7 V vs RHE. Cu III oxide was observed only on the CuO and Cu(OH) 2 electrodes. Interestingly, these two electrodes catalyzed the OER ∼10 times more efficiently than the Cu and Cu 2 O catalysts. By correlating the intensity of the Raman band of Cu III oxide and the extent of the OER activity, we propose that Cu III species provides catalytically active sites for the electrochemical water oxidation. The formation of Cu III oxides on CuO films during OER was also corroborated by in situ XANES measurements of the Cu K-edge. The catalytic role of Cu III oxide in the O 2 evolution reaction is proposed and discussed.